New Old School

Hey. I used to run a 18" JBL 1800 gti sub back in my prime. I just recently got back into sound. I had it on 500 watts. It was in a huge ported box. Used to make my steering wheel shake, and sound like there was a monster in my trunk.

Now.
I got 2 lanzar opti-drive dvc 2ohm 12's in a store bought ported box. I've got a pg 1200rsd amp at 2 ohms. I don't think I'm getting 1200 watts. The subs are rated at 1000/pc, but 600/sub should bring some quad.
Don't get me wrong, my system knocks, but it does not quad. Sounds real good up to about 40 on the HU. Past that it starts to run out of power and break up.
I tested the voltage at the spkr outputs using a meter -no spkr load present- and test tones and got 77v on the PG and 48v on the Boss.

Gains on sub amp up to about 2/3 or a little less most of the time. Gains on highs at min.
Subs sound best crossed at 50hz- 12db slope.
I have 4 ga power and ground with 2nd battery in the rear- stock alt.
Vehicle is chevy astro.
I've got kicker rs65 components in the doors with the tweets in the a pilars powered with ~200 watts/side.
Head unit is jvc 4 volt pre out.

My question....
What should I do -that costs little or no money to upgrade my sub stage